The present invention relates to a method for detecting failure of a relay and, in particular, to a method for detecting failure of a high current relay.
Some vehicle systems, such as Electric Power Assist Steering, traction control systems, blower systems, and the like, require a large amount of current from the vehicle battery. Often, these high current systems pulse width modulate (PWM) the power supplied to the system from the battery. As a result, the battery is typically buffered by way of an electronic control unit (ECU) from these high current loads. The ECU includes a high current relay and one or more bus capacitors. The relay switches or, in other words, connects and disconnects power from the battery to the bus capacitor. The bus capacitor buffers the power supplied by the battery as well as stores and filters high currents. To increase relay life as well as prevent a large inrush of current at vehicle system start-up that may damage the relay, the ECU further includes a pre-charge circuit electrically connected in series between the battery and the bus capacitor and electrically connected in parallel with the relay. At system start-up, the relay is open and the ECU activates the pre-charge circuit. After the bus capacitor is pre-charged, the ECU deactivates the pre-charge circuit and closes the relay connecting power from the battery to the pre-charged bus capacitor.
The high current relay can fail in several ways. For example, the relay can fail in an permanent closed condition, wherein the battery is connected to the bus capacitor, or a permanent open condition, wherein the battery is disconnected from the bus capacitor. Failure of the relay must be detected to ensure a proper operation from the ECU. Previous methods for detecting failure of the relay have several shortcomings. Some shortcomings of the previous failure detection methods include delaying the start-up of the high current system, requiring additional components to detect such relay failures, and failing to verify proper operation of the relay each ignition cycle. Accordingly, there is a desire to provide a method for detecting failure of a high current relay which overcomes the shortcomings of the prior art.